In subsea oil field operations, umbilicals are used to transport fluids, power, signals or data to and from a subsea installation. An umbilical comprises a group of one or more types of functional elements such as multicore low voltage electric cables, optical fibre cables, or hoses for fluid transportation of, for example, gas, water or chemical products such as methanol. These functional elements are assembled together in a helical or S/Z manner and over-sheathed and/or over-armoured for mechanical strength and ballast. It is desirable for a single umbilical to be able to contain as many functional elements as are required for a particular application, for example, as are required for a particular oil field where the umbilical is intended for use.
In the present application, “low voltage” should be understood as rated for a voltage smaller than 3000V, and typically smaller than 1000V. Low voltage multicore cables of subsea umbilicals are generally used to carry single phase power for control purposes and signal transmission. Typically a low voltage multicore power cable used in a subsea umbilical is rated at 250V and 3 A, so around 750 W in power. Low voltage multicore signal cables used in subsea umbilicals are operated at frequencies up to 20 kHz. These multicore low voltage cables typically have a very simple construction. Indeed, because of their low voltage, such multicore cables are not sensitive to moisture ingress, unlike medium and high voltage coaxial power cables (respectively rated at 6 kV to 30 kV and at more than 30 kV) which are known to be very sensitive to water ingress and may develop detrimental defects known as “water tree”, and therefore require additional protective sheaths to prevent moisture ingress.
A problem with known electric cables within subsea umbilicals is that, over an extended period of time, in some circumstances related to the presence of sea water, small quantities of hydrogen appear inside the umbilical structure, and then diffuse inside the electric cables. The presence and the circulation of hydrogen gas inside the electric cables have several detrimental effects.